1. Field of the Invention
The present invention relates generally to a printhead for an electrochemical printing arrangement, and more particularly pertains to an electrochemical printhead which is constructed in an improved manner by utilizing technology developed for the construction of precision arrays of glass nozzles for ink jet printers.
2. Discussion of the Prior Art
Electrochromic printing reactions, also referred to as molecular matrix printing, are well known in the art. In this technology, a superficial aqueous conductive liquid is applied to either regular or precoated paper, and the paper is later subjected to electrical pulses as it passes by an electrochromic printhead.
Numerous molecular species exist in one of two forms, (a) a colorless, leuco state, reduced and having an electron donor, and (b) a colored state, oxidized and being electron deficient. In the colored state the electrons in chromophoric groups are excited to higher orbital positions, and the energy required for the transition is derived from absorption of light. Electrochromic printing is normally accomplished by applying the colorless, leuco molecules to the paper surface, and then selectively removing electrons, in accordance with a desired printing pattern, by using an electrochemical printhead. A printed or colored dot is formed only under the electrodes of the electrochemical printhead to which actuating electrical pulses are supplied because electrons are removed only under those electrode areas. This arrangement achieves a very high print resolution as the size of each resultant printed dot is normally smaller than the cross sectional area of its printing electrode.
Generally, in electrochromic printing the printhead includes two different types of electrodes, active electrodes under which the color reactions are produced and passive electrode(s) which is an electrode(s) of the opposite polarity. The active electrodes are normally print pins of small cross sectional area formed into an array, such as a linear array, and in usage are continuously surrounded by the oxidation products of the electrochromic reaction. The passive electrode is normally constructed as one large common electrode extending along the surface of the printhead, and ideally does not result in any marking effects on the print paper. The chemical reaction at the passive electrode is one of oxygen reduction, or removal of hydrogen from the electrolyte, depending upon the reaction medium and the nature of the electrode. Different types of printheads for electrochromic printing have already been described, and include thin film devices, discrete wires in plastic holders or on etched silicon wafers, and multilayer ceramic head modules.
Ideally, printing should occur with no wearing or consumption of the electrodes because the electrodes are normally formed of a noble metal such as gold, platinum, iridium or rhodium, and the electrode reactions are designed such that the electrode surfaces act only as interfaces for electron exchange and do not participate mass-wise in the printing reaction. However, it has been found that there is electrode corrosion with most electrolyte solutions, even when the electrodes are constructed of the aforementioned noble metals or alloys thereof. This corrosion is caused by the relatively high electrical field between the electrodes (of the order of 300 volts per centimeter) and the relatively high current density (approximately 100 amps per square centimeter) and also because of the formation of chemical complexes between the electrodes and the electrolyte materials. It has been empirically established that mechanical wear, caused by friction between the electrodes and paper, is generally a negligible factor.
Hoffman et al. U.S. Pat. No. 4,019,886 is particularly pertinent to complement the disclosure of the present invention as this patent discloses a method of manufacturing multiple nozzle wafers for use in inkjet printing. In summary, the approach disclosed therein machines ceramic or glass blocks to form two plates of a desired smoothness and dimension, preferably in rectangular form. In a preferred embodiment a single groove is formed the length of one side of the first plate, and cross-slots, deeper than the groove, are formed the width of the same side and intersecting the groove. Slots corresponding to the cross-slots in the first plate are formed the width of one side of the second plate. The groove holds a plurality of glass tubes which may be positioned before or after the two plates are joined. Each slot holds a sealant, such as glass cane, which is entered after the two plates are joined.
The joined plates with tubes and glass cane are then spring clamped in an upright position on a support, and this entire assembly is then exposed to a temperature which is sufficient to melt only the glass cane, which flows by capillary and gravity action, through the groove to provide a complete seal for the tubes, specifically in the area between the cross-slots. After the sealing operation has been completed, the joined plates are gradually cooled and then the area between the slots is sliced into thin nozzle wafers. After one side of the cut wafer undergoes lapping and polishing operations, it is ready for mounting on a back-up plate using techniques such as epoxy bonding, glass sealing or soldering. After mounting, the front side of the wafer is lapped and polished. The wafer thus mounted on the back-up plate is ready for connection to a source of high-pressure fluid.
Although the disclosure of this patent is directed to the construction of wafers for use in ink jet printers, in accordance with the teachings of the present invention the thusly formed wafers may also be utilized for the construction of electrochromic printers.
Bahr et al. U.S. Pat. No. 4,157,554 discloses a multiple electrode printhead for a metal paper printer wherein a printhead body of plastic or glass contains a plurality of closely spaced glass tubes. The glass tubes provide low friction passageways for fine wire electrodes which extend therethrough protruding beyond the printhead body to provide a flexible contact with the metal layer of a recording medium. A pair of feed rolls frictionally engage the individual wires such that when the feed rolls are operated they can adjust the position of the wires to compensate for wear of the ends thereof in contact with the metal layer.
Accordingly, in this arrangement the individual electrode wires are longitudinally movable through the printhead to provide adjustment to compensate for electrode wear. The type of printer and the printing technology are completely different from those of the present invention which utilizes sealed active electrodes in an electrochromic printing operation.